Is oxygen consumption depends on the depth and can we count it?

[Bartlomiej]

During breathing we use about 4% of oxygen

- this sentence is visible in many materials.

I am curious what if we breathing with Trimix 19/30 on the surface, do we exhaled 15/30?
We reason why I ask this question on the rebreathers group is that this divagation is connected with adding air to the circuit on different depths. Can I combine partial pressure with the consumption of oxygen? For example for air:

Depth    inhaled PPO2  exhaled PPO2
0             0.21                 0.17
10           0.42                 0.39
20           0.63                 0.59

 

[tbone1004]

I think it's safer to say in breathing we use about .04ppO2 instead of 0.04 fO2. Your metabolic consumption of oxygen is independent of atmospheric pressure.

Your math above looks correct.

 

[bletso]

I dive a rebreather. My metabolic requirement is a constant 0.9lpm irrespective of depth. It varies a little based on physical activity. Certainly your inhaled PO2 will vary with depth but your exhaled PO2 will vary depending on your physical activity? Further I believe your exhaled PO2 would be lower as you metabolise the same amount of O2 irrespective of depth. (Think of it as a number of molecules of O2 metabolised versus volume of gas PO2) I take it your depth is in meters.

 

[stuartv]

I must be missing something.

The math above shows a delta of:

0 0.04
10 0.03
20 0.04

That doesn't make sense any way you slice it.

And if they were all 0.04, I think it would be wrong.

Since you metabolize the same, regardless of depth, I think the deltas would be:

0 0.04
10 0.02
20 0.1666
30 0.01

Or:

0 0.21 -> 0.17
10 0.42 -> 0.40
20 0.63 -> 0.61333
30 0.84 -> 0.83

Beyond that, I have no idea what "divagation" means.

 

[lermontov]

?
@stuartv
at 30m, if you breath in 21% thats .21 x 4ata = .84ppo2
metabolise 4% so you exhale 17%
at 30m if you exhale 17% thats .17 x 4 ata thats .68 ppo2
at 30m metabolising 4% thats .04x 4ata =.16ppo2

.84 - .68 -=.16
not considering co2

i dont follow your calculations - please clarify

 

[Miyaru]

There's another factor to take into consideration. At the surface, nearly all oxygen is transported by hemoglobin, which is not in a dissolved gas form anymore but bound to the Fe atom. Only a very small percentage of oxygen is transported in the plasma.
At depth, the capacity of hemoglobin doesn't change, but the partial pressure difference for oxygen increases and enables the plasma to transport more oxygen as a dissolved gas.

So I'd say, at deeper depths, the intake of oxygen increases. However, I'm stepping into the domain of @Dr Simon Mitchell who will be able to clarify this much better than me.

 

[stuartv]

?
@stuartv
at 30m, if you breath in 21% thats .21 x 4ata = .84ppo2
metabolise 4% so you exhale 17%
at 30m if you exhale 17% thats .17 x 4 ata thats .68 ppo2
at 30m metabolising 4% thats .04x 4ata =.16ppo2

.84 - .68 -=.16
not considering co2

i dont follow your calculations - please clarify

At the surface, you breathe in 100 units* of gas. 21 of those units are O2. Your body consumes 4 of them. When you exhale, there are only 17 units.

At 30m, you are now breathing in 400 units of gas. 84 units are O2. 1 unit holds 4 times as many O2 molecules as it did at the surface. So, your body only consumes 1 unit of the inhaled gas.

* I am defining 1 unit as 1% of the volume you breathe in and out at the surface. So, if your tidal lung volume (i.e. the volume of gas that goes in and out when you breathe one time) is 10 liters, then 1 unit is 0.1 liters (aka 100 milliliters, aka 100 cubic centimeters, aka 100cc). If your tidal volume is 7 liters, then 1 unit is 0.07 liters (aka 70 milliliters or 70cc). It's just an arbitrary definition of a unit of volume to make the following math simpler.

There's another factor to take into consideration. At the surface, nearly all oxygen is transported by hemoglobin, which is not in a dissolved gas form anymore but bound to the Fe atom. Only a very small percentage of oxygen is transported in the plasma.
At depth, the capacity of hemoglobin doesn't change, but the partial pressure difference for oxygen increases and enables the plasma to transport more oxygen as a dissolved gas.

So I'd say, at deeper depths, the intake of oxygen increases. However, I'm stepping into the domain of @Dr Simon Mitchell who will be able to clarify this much better than me.

I think this is also covered in Deco for Divers, by Mark Powell.

Yes, your body is holding more O2 at depth. But, even though you have more in you, you still only metabolize the same amount. So, once you are at a depth and the O2 on-gassing has achieved equilibrium, the higher ambient pressure still does not change the amount of O2 that is consumed or the amount that is exhaled. When you first reach your max depth, your body will be on-gassing O2 (as well as N2), so there will be some amount of time where the measured O2 consumption would be higher - but I think I remember reading that it is not much at all compared to how much you metabolize. Similarly, when you are ascending, you will off-gas that dissolved O2 and your measured O2 consumption will be less during that period of O2 off-gassing.

 

[tbone1004]

@stuartv that's a bit confusing to think about, it's not wrong, but if you're talking about units, it should be an actual unit instead of a percentage. Mass is consistent with what we use when talking about metabolic consumption.

 

[rjack321]

Would be better to be working in mass, moles, or volume at standard temp and pressure. Percent is not an appropriate unit, saying you are metabolizing 4% is causing lots of confusion and inaccuracies.

 

[stuartv]

@stuartv that's a bit confusing to think about, it's not wrong, but if you're talking about units, it should be an actual unit instead of a percentage. Mass is consistent with what we use when talking about metabolic consumption.

Well, it can be confusing by virtue of using "units", or it can be confusing by having the actual numbers that would come from using previously defined units, like liters and grams or moles. I went with one possibly confusing concept in order to make all the actual numbers very easy to understand.